Remove two unnecessary modifications

1. The _GNU_SOURCE macro definition checking in netbase.cpp, the HAVE_GETADDRINFO_A is enough for using getaddrinfo_a();
2. The interruption points added in net.cpp::ThreadDNSAddressSeed(), using getaddrinfo_a() instead of getaddrinfo() is enough to make the dnsseed thread responsive.

Signed-off-by: Huang Le <4tarhl@gmail.com>